Abstract

Geohistory analyses of Cenozoic marine basins rely heavily on the precision of foraminiferal dating and paleodepth determinations. A rapid method of paleodepth assessment has been devised for the New Zealand region, using a combination of planktonic percentage and the derived depth ranges of dominant benthic foraminifera and a number of diagnostic intrabathyal benthic taxa. This method together with biostratigraphic dating has been applied to approximately 2,000 foraminiferal faunas in 26 drillhole sequences and used to construct paleobathymetric, subsidence, and tectonic curves and maps for Taranaki Basin, New Zealand. Taranaki Basin sequences show a late Cretaceous-early Miocene transgressive phase initiated by Tasman Sea rifting which split New Zealand continental crust off from Gondwana. A pulse of basin-wide rapid subsidence in the late Oligocene to early Miocene resulted from the propagation through New Zealand of an obliquely extensional boundary between the Pacific and Indian Plates. The early middle Miocene heralded a switch to regression as mountains were uplifted along the plate boundary and abundant sediment began to fill the basin, advancing northwestward as a 1-km-high front of giant foreset beds that formed an extensive continental shelf. The abundant sediment masked the middle Miocene to Recent tectonic deformation that occurred in the eastern parts of the basin within the plate boundary zone. Several Neogene episodes of oblique extension and compression, with formation of deep grabens, major domes and thrusts, are identifiable using foraminiferal data.

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